11 wildlife encounters to do soon -- before they possibly disappear

(CNN) — Earth is in the grip of the "sixth great extinction crisis," say some scientists, with up to 2,000 species dying out each year.

These 11 great wildlife experiences could disappear within your lifetime.

So if you've ever wanted to enjoy any of these wildlife encounters, you should probably go ahead and plan your outdoor excursions soon just in case any of these come to an untimely end:

1. Witness a great animal migration

Ancient rock art images of animals moving across the African savannah indicate that humans have been marveling at animal migrations for more than 20,000 years.

But poaching, agriculture, war and deforestation are encroaching on wildlife corridors around the world, placing these animal movements -- from Africa's wildebeest migration to the annual pilgrimage of North American bison -- under threat.

How to do it: GAdventures pairs good wildlife access with creature comforts on its Kenya Safari Experience tour, taking in Kenya's Lake Nakuru, Amboseli National Park and the Masai Mara, where Africa's wildebeest end their migration.

3. Count the stripes on a tiger

Tigers once roamed freely across Asia. Today the world's most endangered big cats have disappeared from 90% of their historic range. Just six of nine known tiger species are thought to remain, with an estimated 3,200 animals left in the wild.

While it's extremely rare to spot the elusive beasts in their natural habitat (the South China tiger, for example, hasn't been seen since the early 1970s), protected reserves in India and Nepal provide the best opportunities.

How to do it: Intrepid Travel offers a Chitwan Extension tour to Nepal's Chitwan National Park from Kathmandu.

Once a royal hunting reserve, Chitwan is home to elephant, rhino, crocodile, monkeys, deer and about 100 Bengal tigers.

4. Swim amid live coral on the Great Barrier Reef

Tracts of the world's largest reef system are in danger.

Tourism Queensland.

The Great Barrier Reef, the world's largest reef system, is in peril.

Temperature fluctuations are primarily to blame for the disappearance of half its coral since 1985 and the Australian Institute of Marine Science predicts that less than a quarter could be left within a decade.

Following plans by the Australian government to industrialize parts of the reef, the World Heritage Committee passed a recommendation to consider listing the UNESCO site as "in danger" at a meeting in June 2014.

How to do it:Calypso Snorkel and Dive offers daily snorkeling and diving day trips to the reef from the Far North Queensland resort town of Port Douglas.

5. Track gorillas in the African mist

Endemic to the forests of central Africa, both species of the world's largest primate have been decreasing in numbers for decades largely due to poaching and disease.

Worryingly, a 2010 United Nations report suggested that these gentle giants of the forest might disappear from large parts of the Congo Basin, one of their primary habitats, by the mid-2020s.

How to do it: Wilderness Collection offers gorilla tracking opportunities split between two luxury Odzala camps in an area of the Republic of the Congo that's home to a high density of western lowland gorilla.

Chimpanzee, leopard, golden cat and a large number of other primate and bird species are also present.

6. See the Galápagos as Darwin saw them

Remaining virtually undiscovered for centuries did much to preserve the living showcase of evolution described by Charles Darwin in "The Origin of Species." But Ecuador's best known ecosystem remains fragile.

The last remaining Pinta Island tortoise, fondly known as Lonesome George, died in 2012, adding to the list of several other species that have died out since Darwin's 1835 visit.

How to do it: While it's possible to fly to Galapagos from mainland Ecuador and visit several islands independently, you can only reach the bulk of protected habitats on heavily regulated organized cruises.

You can book a last-minute, week-long cruise organized in the capital Quito or Puerto Ayora on the island of Santa Cruz, or for more convenience, book ahead through an international company such as Ecoventura.

7. Hang around with orangutans

Sharing 98% of our DNA, orangutans are one of the closest relatives to man, though if predictions are accurate, Asia's only two great ape species could be extinct in the wild within the next decade.

Conservation groups such as Borneo Orangutan Survival have bolstered numbers through rehabilitation and release programs, but the illegal pet trade, trafficking and habitat destruction still present the greatest threats to the survival of orangutans on the only two islands they inhabit: Borneo and Sumatra.

How to do it: Gecko's Adventures include a visit to a rehabilitation center in Malaysian Borneo on their Orangutanland tour.

The trip includes a cruise down Kinabatangan River, where you might spot orangutans, proboscis monkeys, macaques and elephants in the wild.

8. Watch sea turtles nest

Scientists have been debating for decades why -- not to mention how -- sea turtles return to the same beach every year to lay their eggs.

But the phenomenon may soon be a thing of legend thanks to the ravages of intense egg collection, fisheries bycatch and light pollution, which have forced species such as the leatherback, hawksbill and green turtles onto endangered lists.

How to do it: You can sign up for a volunteering stint with an organization such as Earthwatch during which you'll help safeguard turtle eggs and assist new hatchlings.